As long as you have downloads enabled in your online galleries, when your client favorites photos in their online gallery they have the ability to download just those favorite photos on their own!
Your clients are able to favorite photos by clicking on the "heart" on the image.
After your client has started favoriting photos, they can click on the "Download" icon at the top of the gallery.
They can select the "Favorites" folder, and add any other folders that need to be downloaded, and then enter in their e-mail address. After clicking "Download photos," your client will then be emailed the download link 🎉